In psychotherapy of "gender identity disorder" in Japan, psychiatrists assess the "sex of mind" of their clienteles. In this process, medicine first treats the invisible "sex of mind" as more essential than the visible sexed body, then as visible in the clientele's looking, behaviors, talks etc. The aim of this article is to examine how medicine treats the sex category according to this paradoxical logic, using the review of medical discourses and scripts based on interviews of "gender identity disorder" clientele. Through this process of description of the medical practice, it is showing how, by seeing the clientele's display as trans-situational clues of the "sex of mind", this practice may be actually seeing very "sex of mind".